1. Field of the Invention.
The present invention relates to a method for the accurate measurement of the density of a sample.
2. Description of the Related Art.
Of the physical measurements which have to be effected in the context of industrial processes, that of density figures among the most frequently necessary.
To that end, manufacturers market a range of densimeters based on various principles, all of which have advantages and disadvantages.
A densimeter which can be used in a satisfactory manner to measure the density of a sample is equipped with a measurement cell comprising the following elements:                a substantial enclosure which has very good heat conductivity and which defines at its inner portion a measurement chamber closed by a stopper;        a U-tube which is kept earthed and is secured to the stopper and which is to be filled with the sample being analyzed and extends inside the measurement chamber;        an insulated conductive reading plate maintained at a difference in potential relative to the U-tube and secured to the stopper in a position facing that tube in order to define a capacitor therewith;        an electromagnetic excitation winding mounted in a housing of the enclosure at right-angles to the ferromagnetic member; and        means for operating the winding as well as means for reading the voltage at the terminals of the capacitor.        
The U-tube of the cell of a densimeter of that type comprises, on the one hand, a central limb equipped with a ferromagnetic member at its middle portion and, on the other hand, two lateral limbs projecting outwardly from the measurement chamber to permit the injection of the sample being analyzed into the U-tube and the evacuation of that sample.
The principle of measuring the density of a sample by means of such a densimeter consists in causing the U-tube to vibrate at resonance frequency and in determining that frequency from the variations in the voltage at the terminals of the capacitor.
The resonance frequency enables the density of the sample being analyzed to be calculated to a first approximation on the basis of a standard equation known per se and from a preliminary calibration of the densimeter.
Such a calculation is, however, admissible only if an accuracy greater than e−4 g/ml is not required because it does not take into account the damping effect caused by the viscosity of the sample.
Various algorithms have already been proposed to take that viscosity parameter into account but they all have the disadvantage of requiring several measurements, which makes them both time-consuming and onerous.